Tree borers are insects whose larvae tunnel into the wood of living trees, feeding beneath the bark and destroying the tissues that move water and nutrients through the trunk and branches. They belong to several insect families, including beetles, moths, and even wasps, and they are one of the most destructive categories of tree pests in North America. A single borer larva can girdle and kill a small tree, and heavy infestations can bring down large, mature trees within a few years.
What Tree Borers Actually Are
The term “tree borer” isn’t one species. It’s a catch-all for dozens of insects that share the same destructive habit: their larvae chew tunnels into living wood. The adult insects lay eggs on or near bark, and the hatched larvae bore inward to feed. The major groups break down by insect family.
Flatheaded borers are beetle larvae in the metallic wood borer family. Their bodies are flattened behind the head, giving them their name. The most notorious member is the emerald ash borer, an invasive species from Asia that has killed hundreds of millions of ash trees across North America. A native relative, the flatheaded appletree borer, attacks a wide range of hardwoods including oak, maple, ash, and apple.
Roundheaded borers are larvae of longhorned beetles. They tend to be cylindrical with rounded head capsules. The banded ash borer and redheaded ash borer both fall into this group. Despite its name, the redheaded ash borer more commonly develops in fruit trees than in ash. The banded ash borer targets ash, oak, elm, and linden.
Clearwing borers are moth larvae. The lilac/ash borer is the most common wood borer found in ash trees across much of the western United States. Adult clearwing moths often mimic wasps in appearance, which can confuse homeowners who spot them near infested trees.
Bark beetles are tiny beetles (around 3mm long) that tunnel just beneath the outer bark. Ash bark beetles, for instance, create characteristic branching galleries in the inner bark. Horntail wasps, like the pigeon tremex, are wood-boring wasps that typically target hardwood trees already in advanced stages of decline, including elm, maple, and ash.
How Borers Damage a Tree
The real destruction happens out of sight. Just beneath the bark lies a thin layer of living tissue: the phloem (which carries sugars made by leaves downward), the cambium (the growth layer), and the outer xylem (which carries water upward from the roots). Borer larvae chew winding tunnels through these layers, severing the tree’s internal plumbing. When enough of this tissue is destroyed, the trunk or branch becomes girdled, meaning the flow of water and nutrients is cut off in a complete ring around the wood.
A girdled branch dies. A girdled trunk kills the entire tree. The emerald ash borer’s larvae feed on inner bark from roughly June through October each year, and because multiple generations can overlap, the damage compounds quickly. Infested black, green, and white ash trees in studied populations suffered over 99% mortality, with 60% of those deaths occurring within just five years.
Signs of a Borer Infestation
Borers are hidden insects, and infestations often go unnoticed for years. External symptoms typically don’t appear until about two years after the initial attack, and most infestations aren’t detected until around four years in, when the tree visibly starts to decline. Knowing what to look for can buy you critical time.
Canopy thinning and dieback. The first visible symptom is usually thinning foliage in the upper canopy. Branches at the top of the tree may produce fewer leaves, and individual limbs may die back entirely. This happens because the tunneling below has cut off water supply to the highest points first.
Exit holes in the bark. When adult borers finish developing, they chew their way out of the tree, leaving small holes in the bark. The shape of these holes is a useful identification clue. Emerald ash borers leave distinctive D-shaped exit holes about 1/8 inch wide. Roundheaded borers leave round holes. Clearwing moths leave slightly larger, oval openings, sometimes with pupal casings sticking out.
Frass and sawdust. Frass is a fine mixture of sawdust and insect waste. You may see it collecting in bark crevices, at the base of the trunk, or packed into tunnels visible where bark has split or been removed. If you peel back a section of loose bark on a declining tree, you may find the serpentine, S-shaped galleries packed with frass that are characteristic of emerald ash borer larvae.
Bark splitting and woodpecker activity. As tunneling weakens the wood beneath, bark may crack or split. Heavy woodpecker activity on a tree that previously didn’t attract them is another red flag. Woodpeckers feed on borer larvae, and their flaking of bark (sometimes called “blonding” because it exposes lighter wood underneath) can indicate a significant infestation below the surface.
Trees Most at Risk
Borers tend to target stressed trees. Trees weakened by drought, compacted soil, root damage, recent transplanting, sunscald, or construction activity near their root zone are far more attractive to egg-laying adults than vigorous, healthy trees. Many borer species can detect the chemical stress signals that weakened trees emit.
Ash trees face the greatest threat due to the emerald ash borer, which attacks even healthy ash with devastating efficiency. But oaks, maples, elms, fruit trees, birches, and lindens all have their own suite of borer species. Newly planted trees and young trees with thin bark are especially vulnerable because their bark offers less of a physical barrier to larvae.
Prevention Through Tree Care
The single most effective defense against most borers is keeping your trees healthy. Vigorous, well-watered trees produce chemical defenses and sap flow that can drown or repel young larvae before they establish. The University of California’s integrated pest management program emphasizes that excellent cultural care and optimal growing conditions are critical to avoiding most borer problems.
Practical steps that reduce borer risk:
- Water consistently. Deep, regular irrigation during dry periods prevents the drought stress that attracts borers. This is especially important for recently planted trees and during heat waves.
- Protect trunks from injury. Wounds from lawn mowers, string trimmers, construction equipment, or sunscald create entry points and stress signals. Whitewashing trunks or wrapping them with heavy paper can prevent sunburn on thin-barked species. If you wrap trunks, remove the wrapping periodically to inspect for damage.
- Avoid soil compaction over roots. Parking vehicles or storing heavy materials over a tree’s root zone compresses soil and suffocates roots, slowly stressing the tree.
- Prune properly. Remove dead or dying branches promptly, as these attract secondary borers that can then spread to healthy wood. Avoid heavy pruning during the growing season when adult borers are active and seeking egg-laying sites.
Insecticide Treatment Options
For high-value trees facing confirmed borer pressure, particularly ash trees in areas with emerald ash borer, chemical treatment can be effective if started before the tree has lost more than about half its canopy. Four active ingredients are commonly used against borers, each with different application methods and timelines.
Trunk injection with emamectin benzoate is considered the most effective option for larger trees. It requires a professional with specialized equipment, provides at least two years of protection per treatment, and is best applied between spring bud break and late June or early July. Soil-applied imidacloprid works well for smaller trees (less than about 6 inches in trunk diameter). You mix it with several gallons of water and pour it onto the soil around the trunk base in spring, shortly after bud break. It protects for one season and needs annual reapplication.
A bark spray using dinotefuran is the only option that can be applied directly to the trunk surface and still move systemically through the tree. It’s useful when soil application isn’t practical, though it also lasts only one season. Neem-derived azadirachtin, a naturally sourced option, requires professional trunk injection and typically provides two years of protection during early infestations, dropping to one year when borer populations peak.
Timing matters enormously with all of these treatments. The goal is to have the insecticide present in the tree’s tissues before adult borers emerge, feed, and lay eggs. Applying too late in the season means larvae are already established beneath the bark, where systemic treatments are less effective.
Biological Controls
Beneficial nematodes, specifically Steinernema carpocapsae, are microscopic worm-like organisms that parasitize and kill borer larvae. They’re particularly effective against caterpillar-type borers like the peach tree borer, with field studies showing around 70% efficacy. Nematodes are applied as a water suspension and work best when they can reach larvae in their tunnels, so they’re most practical for borers near the soil line or in accessible parts of the trunk.
For emerald ash borer specifically, several parasitoid wasp species have been released as part of federal biocontrol programs. These tiny, non-stinging wasps lay their eggs on or near borer larvae, and the wasp offspring consume the borer. These programs operate at a landscape scale and aren’t something individual homeowners can purchase, but they’re gradually establishing in many infested regions and helping to reduce borer populations over time.
When a Tree Can’t Be Saved
If a tree has lost more than half its canopy to borer damage, treatment is unlikely to restore it. Trees in this condition have suffered too much internal tissue destruction to recover even if the borers are eliminated. Dead and heavily infested trees also become structurally dangerous. Borer tunneling weakens wood from the inside, making limbs and trunks prone to sudden failure. Removing a severely infested tree promptly also reduces the local borer population and protects nearby healthy trees from heavier attack pressure.